ABSTRACT
Water on Earth is constantly moving. Water evaporates from the soil and open water surfaces as a result of solar and wind energy. In addition, plants draw water up from the soil and release it to the atmosphere through the stomata on their leaves, a process called transpiration. The processes of evaporation and transpiration together are called ‘evapotranspiration’ (although in daily language, the short term evaporation is generally used to include transpiration as well). The amount of water in the atmosphere increases through evapotranspiration, but decreases again through precipitation. Within the atmosphere, water vapour moves around the globe according to complex patterns, so water that evaporates in one place does not necessarily return as precipitation in the same place. The amount of water on land increases as a result of precipitation but decreases as a result of evapotranspiration. Since precipitation on land exceeds evapotranspiration (not on a daily scale, but in the long run), there is a water surplus on land, which leads to run-off. Run-off water from land finally ends up in the ocean. While lands have a precipitation surplus, oceans have an evaporation surplus. Overall, there is net transport of water from oceans to land through the atmosphere. It is brought back from land to ocean through run-off. Run-off occurs partly through overland flow (rivers and streams) and partly through groundwater flow. The volume of water on Earth remains more or less equal.
